AstraZeneca returns treatment rights to Rigel

Shares of Rigel Pharmaceuticals Inc. plummeted Tuesday before markets opened and after the drug developer said partner AstraZeneca will not seek regulatory approval for the potential rheumatoid arthritis treatment fostamatinib.

AstraZeneca also will return the compound's rights to Rigel, which said it will take a couple months to figure out the next step for the potential drug.

Rigel doesn't have any approved products, and fostamatinib is its most advanced experimental drug. In 2010, the company licensed fostamatinib to AstraZeneca, a British company that is one of the world's largest drugmakers.

Rigel shares also slid in April, after the companies announced mixed results from a late-stage clinical trial of fostamatinib.

Rigel, based in South San Francisco, Calif., said Tuesday that additional studies of the drug also yielded mixed initial results, with patients taking one of the treatment regimens in the study failing to show a statistically significant difference from those taking a placebo, or fake drug.

Shares of Rigel fell $1.13 cents, nearly 25 percent, to $3.39 Tuesday in premarket trading. The stock had already dropped about 30 percent so far this year, as of Monday's close.